1. Field of the Invnetion
This invention relates to improvements in driver circuits and methods for polyphase dc motors, and more particularly to improvements in circuitry and methods for sensing currents in power driver transistors for polyphase dc motors, and still more particularly to improvements in circuits and methods for improving the accuracy of current sensing in polyphase dc motor driving circuits.
2. Relevant Background
Typical hard disks, such as used in personal computers, CD-ROM drives, and like applications typically are rotated by polyphase dc motors. A polyphase dc motor, and, in particular, a three phase dc motor, which is widely used in such applications, has three coils connected between respective nodes A, B, C, and a common center tap node, CT. As current is commutatively applied to flow through successive sets of the coils, a rotor is caused to rotate in synchronism with the commutation frequency. Typically the current is applied to flow through the coils in a sequence as follows: AC, BC, BA, CA, CB, and AB.
The currents are controlled by a current driver circuit, which typically includes three current paths, each including two driver transistors. The driver transistors may be, for example, FET devices. Each current path is connected between V.sub.CC and ground with the coil nodes A, B, and C connected respectively between each set of driver transistors. The driver transistors between the coil nodes and V.sub.CC are generally referred to as the "high side drivers" and the transistors connected between the coil nodes and ground are generally referred to as "low side drivers." A commutation circuit sequentially applies control voltages to the gates of the respective high and low side drivers to cause currents to flow in the motor coils in the sequence described above.
Typically, the currents in the respective current flow paths of the driver circuit are desirably equal, but due to variations in fabrication processes, mask alignments, and so on, the FET driver transistors in the respective paths typically have tolerances, for instance, in their channel widths or lengths, resulting in inequalities in the respective currents. As a result, noise and other undesirable effects result. Such unequal currents also reduce the precision of the motor speed. As densities of hard disk drivers become greater and greater, such imprecisions result in limitations on the densities that might be achieved in a particular hard disk drive.
In HDD-servo applications it is customary to use an expensive high tolerance external sensing resistor as a mean to measure the motor current. Recently, it has been suggested to replace the external sensing resistor with an internal FET device, which generally saves cost and printed circuit board area.
Thus, to detect the currents flowing in the power transistors in the current flow paths of the motor, FET devices may be connected as current mirrors to mirror the current in the power transistors, generally the low-side driver transistors. The mirror transistors are typically made small compared to the size of the power transistors, ratios of 15/1 being typical. It is, however, desirable to make the mirror transistors very small in comparison to the power transistors, but design tolerances result in greater imprecision amongst various mirroring transistors as the size ratio increases.
However, the use of such FET mirror devices often produces poor accuracy because of the large size difference between the power devices and the mirror devices. Normally current flowing through one transistor can be mirrored to another transistor with relatively good accuracy if the devices are of similar size. In motor driver applications, for power FET devices where there are amps of current flowing in the primary devices, it is desirable to use a large ratio (on the order, for example, of 1500:1) between the power FET and the mirror device. If a smaller ratio were to be used, a large amount of power would be wasted in the control circuitry because of the large current in the mirror device. However, with a mirror ratio at the order of 1500:1, the mirror accuracy will not be very good, and can be expected to vary on lot and device basis.
Moreover, in a 3-phase motor, for example, current sensing technology using FET devices requires mirroring of currents from three different current devices. Since variations among the power devices generally exist, the motor current in each of the three phases varies as the motor commutates from phase to phase. In addition to causing torque ripple, a varying motor current may create drive resonances and audible drive noise in the 2-4 kHz range, which is a typical commutation frequency.
What is needed, therefore, is a circuit and method that provide phase current accuracy through mirror transistors of relatively small size, to remove a source of torque ripple, audible noise, and motor inaccuracies.